In a superhet receiver, also known as a superheterodyne receiver, an incoming modulated RF (radio-frequency) carrier signal is converted to an IF (intermediate-frequency) carrier value for additional amplification and selectivity prior to demodulation. Often, a superhet receiver is coupled with a pulse count FM (frequency modulation) or FSK (frequency shift keying) demodulator for recovering a desired audio-frequency signal.
Standard techniques for designing a superhet receiver with a pulse count demodulator include the use of hard limiting amplifiers instead of linear IF amplifiers and AGC (automatic gain control) schemes to minimize the design complexity. Such a technique is well known for FM or FSK receivers using a higher IF frequency.
FIG. 1 illustrates a block diagram of a conventional radio IF and demodulator with a moderately low IF and pulse counting demodulator. A modulated RF input is received by an amplifier 10. An output of the amplifier 10 is coupled to a mixer 20. The mixer 20 is coupled to provide its output to an IF filter 40. The modulated RF signal is combined with a local oscillator signal to produce a modulated IF signal as output of the IF filter 40. The IF signal is comprised of a modulated signal, which is the signal the receiver is designed to recover, and the carrier signal. The frequency of the carrier signal is the IF frequency. The combination of the amplifier 10, the mixer 20 and the IF filter 40 is conventionally referred to as a superhet receiver. The IF filter 40 is coupled to an IF limiter 50 via some means of ac coupling. The IF limiter 50 receives the IF signal, removes the amplitude information from the IF signal and outputs a square wave IF signal. The remaining characteristics of the square wave IF signal are the same as the IF signal input into the IF limiter 50.
The frequency modulation is found in the period of the output pulses of the IF limiter 50. This “pulse count demodulator” function is also known as a “frequency to voltage converter.” The pulse count demodulator comprises a monostable 60 and a data filter 70. The IF limiter 50 is coupled to the monostable 60. The monostable 60 is triggered once for every cycle at the IF frequency. In other words, the monostable 60 will generate one output pulse for each cycle at the IF frequency. The output of the monostable 60 has a strong component at the carrier signal IF frequency and a weaker component at the modulated signal modulating frequency. The resulting output signal spectrum is dominated by the IF frequency whereby the mean rate pulse of the monostable 60 is the same as the IF frequency.
The output pulses of the monostable 60 are received by the data filter 70. The data filter 70 filters out the IF frequency components and passes the modulation frequency components thereby recovering the desired audio frequency signal. When designing the data filter 70, compromises must be made to take into account the following: the lower the IF frequency the stronger the recovered modulation component but the lower the IF frequency the less the difference between the modulation component and the IF frequency. A realizable data filter needs to remove the IF frequency while passing the modulation frequency.
A known limitation of the technique described in relation to FIG. 1 is that the output of the monostable contains more unwanted energy at the IF frequency than wanted energy at the modulation frequency. In other words, the output from the monostable is dominated by the IF frequency which makes the process of filtering out the IF frequency while passing the modulation frequency much more difficult. For proper filtering to occur, the IF frequency must be at least twice the bit rate of the transmission. For example, with a bit rate of 1 Mbits per second, the IF frequency must be at least 2 MHz. Many applications exist that require the use of a lower IF frequency, often a frequency which is of the same order or lower than the bit rate of transmission.
What is needed is a superhet receiver and demodulator circuit which is capable of recovering FM or FSK modulation from a lower IF frequency, an IF frequency which is often of the same order or lower than the bit rate of transmission.